If discard patterns were found to be more homogeneous across regions than fisheries, a fisheries-by-fisheries approach to discard reduction might be more relevant. All biological data such as the numbers and weights where available of discarded and landed species were summarized by region, subarea per region i.
A summary of a detailed comparison of each of the national discard sampling programmes is provided in Table 1. Sampling allocation schemes, species identification and measurement procedures, and raising units of national discard sampling programmes part of the European DCF.
For example, how the units of the sampling frame e. Biological data were collected on a haul-by-haul basis and, for the majority of samples, consisted of landings and discard observations of commercially valuable species including invertebrates such as crustaceans, molluscs, and cephalopods.
Numbers discarded, numbers landed when these were registered , and lengths cm were recorded. To our study, numbers rather than weights were used, because species weights of catch and discards were not recorded in all national sampling programmes owing to the challenge of obtaining accurate weight measurements at sea.
All numbers were raised to the haul level if a subsample was measured; based on the proportion between the total and the sampled fraction and subsequently to the trip level based on either the proportion of sampled fishing operations or fishing time; see Table 1 and ICES, for details.
These raised numbers of landings and discards per species per sampled trip were standardized by sampled fishing time i. DPUE , as the numbers landed or discarded per hour per sampled trip. From all sampled trips, an average and an s. For fewer trips, a greater gap is needed for a similar significant difference. As a measure of the variability of discard rates and ratios across fisheries or regions, we computed the coefficient of variation for discards rates and ratios by fisheries and region.
To calculate the respective CVs, the average and the s. The comparisons of discard rates and ratios were done specifically for towed-gear fisheries that operated under different national flags. Pelagic fisheries which require specific sampling procedures were not considered in this study.
Acknowledging the different species composition of discards in the Mediterranean Sea, for this region, the following list was nominated in accordance with the above criteria: bogue Boops boops , 10 cm according to national legislation in Greece , red mullet Mullet barbatus barbatus , 11 cm , and deep-water rose shrimp Parapenaeus longirostris , 2 cm carapace length.
National discard sampling programmes are not standardized at the European level and exhibit differences in the way vessels are selected for observation, the level of detail that is recorded during biological sampling e. Notwithstanding the above, sampling effort and landings and discard rates were compiled for 15 towed-gear fisheries and 11 major European fishing regions 22 ICES Divisions and 5 Mediterranean geographic subareas; see Helmond and Uhlmann, for details.
Among these classified fisheries, there were differences in fleet size, fishing effort, and sampling effort between countries Table 2. The variability in sampling effort is reflected in the precision of the estimates Figure 1. Discard and landings rates with inferential error bars of commercially valuable species across fisheries for a given region inter-fishery, top row and across regions for a given fishery inter-region, bottom row of plots : a Atlantic cod, b haddock, c European hake, and d European plaice, when combined across countries and ICES Divisions, and e red mullet, f deep-water rose shrimp, and g bogue, when combined across countries fishing in the Mediterranean Sea.
To improve visibility of bar plots, the y -axis scaling was broken where large differences between landings and discard rates existed. Discard rates of cod and haddock Figure 1 a and b were generally lower than those of hake and plaice Figure 1 c and d.
Some of the Mediterranean species such as red mullet and deep-water rose shrimp exhibited the lowest rates Figure 1 e and f. In general, there were distinct patterns when comparing species-specific discard rates across fisheries and regions Figure 1.
For example, discard rates of Atlantic cod were found to be homogenous across fisheries, but were higher in the Skagerrak than in other areas Table 3 ; Figure 1 a. For haddock, differences of discard rates between regions were larger than between fisheries Table 3 ; Figure 1 b.
Hake discard rates were relatively low and similar between different fisheries and regions, except for bottom-otter trawlers targeting fish in the Celtic Sea or crustaceans in the Bay of Biscay Table 3 ; Figure 1 c. For plaice, the differences of discard rates between fisheries seemed to be of the same order of magnitude than between regions Table 3 ; Figure 1 d.
Notably, discard rates of plaice differed greatly between beam and otter trawls in the North Sea, but were much more homogenous across fisheries in the Irish Sea Table 3 ; Figure 1 d. In general, otter trawlers targeting crustaceans were observed to discard the majority of the cod, hake, and plaice compared with those targeting fish Figure 1 a—d.
Both discard rates and ratios were lower in the Mediterranean Sea than in other regions Tables 3 and 4 ; Figure 1 e—g. In the Mediterranean Sea, landings rates largely exceeded those of discard rates Figure 1 c, e, and f , except for bogue Figure 1 g.
Discard ratios of hake were more homogenous than discard rates Tables 3 and 4. The discard ratios of hake varied more in the Mediterranean Sea than in the Celtic Sea, where hake discards exceeded landings, although it is a target species by the fleet operating there Table 4 ; Figure 1 c.
Our study highlights the variability of species-specific discard rates at a European scale. A stark contrast was observed between rates in the Mediterranean Sea and the other fishing regions. Further, we found that discard rates were more homogeneous across fisheries than regions, suggesting that discard management measures may be devised at a regional level; for example, by removing quota and catch composition rules e. EEC, b and incentivising the use of more selective gears.
In any case, differences in discard rates between species will also require species-specific approaches to discard reduction such as improvements to gear selectivity parameters.
Although undersized hake, for example, are being caught by demersal otter trawlers, the proportion in weight of discarded individuals is small Damalas and Vassilopoulou, The fast-growing, smaller, and highly diverse fish fauna Stergiou et al.
Apart from removing quotas and catch composition rules, incentives to increase the use of more selective gears may be another option to reduce discards. One of the more selective gears and fishing methods in our study, where the majority of the target catch was landed, were Danish seines catching cod in the Baltic Sea and plaice in the North Sea Figure 1 b and d.
Scottish seines seem equally selective for other target species such as megrim Lepidorhombus whiffiagonis ; Borges et al. Some gears and methods have become more selective in recent years beyond the period investigated here in some areas e.
Kattegat and Skagerrak , and their uptake throughout the fishing community was partly promoted by incentives such as an increased quota share, access rights, and more fishing days Madsen and Valentinsson, This will also facilitate the standardization of discard sampling approaches ICES, Another shortcoming, which hampered our analysis, was the inability to combine both raw data of fishing effort and catch statistics, partly due to the requirements of a data harmonization software for species weights which were not routinely collected in all programmes Anon.
The lack of recording a species' subsampled and total weight in some sampling programmes precluded the use of the COST software Anon. Data incompatibility and confidentiality were also the reasons why we ended up contrasting aggregated data at the sampled trip as opposed to the fleet level. However, some inferences from patterns at the trip to the fleet level are possible. For example, the greater variability in discard rates between regions than fisheries may be a consequence of the region-specific quota and landings regulations, if acting as the main drivers of discarding Catchpole et al.
For example, the main reason for discarding cod by Danish otter trawlers in the Baltic Sea was catches below MLS, whereas in the North Sea and Eastern Channel cod discards were also driven by a lack of sufficient quotas Catchpole et al. Nevertheless, the interpretation of differences between discard rates based on the available dataset is difficult for two reasons: first, not all species are caught and discarded in significant amounts in all regions; thus, for each region, we did not necessarily have data on the same species from all countries.
Discards can be fish of any size and species, shellfish or benthic debris. To the catching sector, the traditional understanding of discards has been the small fish under the minimum landing size MLS that are brought onboard and then thrown back as they are not allowed to land them by law. If we look back to the s in the North Sea trawling and seine net fishery, before the advent of shelter decks, fish reception hoppers and conveyers, the skipper had a much better impression of what was being discarded as he could see his crew having to physically shovel up the catch, select fish to retain then dump the rest over the side.
Nowadays it is very easy for the crew and skipper to have a false impression of the proportion of catch that they are discarding. On modern vessels the part of the catch that is being discarded does not need to be handled at all, only the retained catch is picked off the conveyor. The discards are disposed of, relatively unnoticed, through a hatch in the side of the boat. There are vastly different discard issues dependent on the type of fishery and the different types of gear used.
There has been a lot of publicity recently on the discards issue, particularly with UK vessels which use all forms of towed gear. Regulations dictate the amount of fish Total Allowable Catch or quota , the proportion of species and the size of fish minimum landing size that can be landed.
Fish may also be discarded for economic reasons - this could be because there is no commercial market, or they are of low value, or they are damaged. There can be numerous categories of discards but four basic classifications are particularly relevant to UK fisheries. Only records from Spain 18 records , Italy 54 records , and Greece records were available. The graphical representations enable to instantaneously evaluate if fishermen were discarding mature or immature individuals as well as below or above MCRS.
Despite that some differences in LFM may have been reported across the basin for a given species, we assumed the same LFM value for each species independent of the GSA.
Specifically the median LFM for each species was calculated based on the data reported in Tsikliras and Stergiou Table 1. For crustacean species not reported in Tsikliras and Stergiou LFM was calculated by reviewing other available scientific literature Supplementary Materials, Table S1.
The median values were used instead of mean, because they are not influenced by the outliers in the dataset Zar, If LFM differed between genders, the more conservative larger median value was used. For the sake of simplicity, in the presentation of the results we show a the most commercial species, b some common bycatch species with commercial interest, and c some common elasmobranch species in the bottom trawl fisheries. Nevertheless, full records that we collected are listed in the Supplementary Materials accompanying this paper.
All species specific discard ratios that derived from the literature review as concerns the EU Mediterranean bottom trawl fisheries are listed in Table S2 of the Supplementary Materials. Figure 2 summarizes the compiled published information on discard ratios for the most frequent species found in our database, for the whole Mediterranean. These include nine bony fish, three elasmobranch, and three decapod species.
Both target species, such as Merluccius merluccius hake , Mullus barbatus red mullet , Aristeus antennatus red shrimp , and some abundant commercial bycatch species such as Boops boops bogue , Trachurus sp. The box-plots highlight the highly fluctuating discard ratios as a characteristic of these fisheries; great range in discarding was observed among and within species, in different areas from the western to the eastern Mediterranean as well as for target and commercial bycatch species Figure 2.
In addition, the large number of outliers Figure 2 is probably in close relation to the latter, as some outliers of high discard ratio can be attributed to low captures or small sizes of a species in a given season, depth stratum, etc. Figure 2. See Table 1 for species codes.
The above information is also analyzed by country and GSA in Figure 3 bony fish and Figure 4 decapods and elasmobranchs. High variability in discarding is observed among countries and GSAs. Even though discard ratios for some species were similar and almost always negligible across the basin, regional variations were observed for others Figures 3 , 4. Bogue, a coastal species, is a special case in the Mediterranean, since it was almost completely discarded in the west Spain but showed commercial importance in the east Greece.
The other coastal species, Pagellus erythrinus red pandora , also showed different discard ratios depending on the areas, i. The discard ratios of greater forkbeard, blue whiting, and Lepidorhombus boscii four-spot megrim were quite homogeneous in the western and eastern areas with the exception of Croatia where the ratios were generally higher Figure 3. Figure 3. Figure 4. For crustaceans, the main targets of the shelf-break to middle slope trawl fisheries, discards were almost null for red shrimp and deep water pink shrimp in the Western and Eastern areas respectively, and very low for Norway lobster in almost all areas Figure 4.
However, taking into account the data available, no clear seasonal patterns were observed. Figure 5 summarizes the L 50 s for each country and GSA for the same species presented earlier, with the exception of Norway lobster and velvet belly lantern shark for which no records of lengths at discarding were retrieved. Within species geographical differences in the lengths of discards were observed; however they were usually not as pronounced as the differences in discard ratios.
Another interesting outcome of the graphs is that the retention sizes of the target species were very small compared to bycatch species even if they concerned larger species with larger maximum length. For example, the median L 50 for hake is 10—17 cm depending on the country and the median L 50 for horse mackerels is 18—21 cm Figure 5 , Supplementary Materials Table S3 , despite that hake and horse mackerels LFM Lmax are Figure 5.
MCRS blue horizontal lines and lengths at first maturity red horizontal lines are also indicated. Bottom trawling produces the bulk of discards in the Mediterranean fisheries Tsagarakis et al. Thus, it is not surprising that there is a large amount of information across the basin as concerns discards of the bottom trawl fisheries.
Because of the nature of discard research, there are many interesting discard studies as gray literature reports, working documents, national reports to authorities, etc. This paper makes available some of this information in a synthetic approach, which is quite important for future research and management, e. As expected, the information was more frequent for the commercially most valuable and most abundant species in the bottom trawl fisheries, thus, inevitably, we chose to focus our presentation on these.
Discards studies in general consider and focus their estimations on valuable commercial species; however, the multi-species nature of catches in the Mediterranean, driven by the diversity of assemblages and bottom substrata de Juan et al. This is important for the sustainable exploitation of the resources in the context of the Ecosystem Approach to Fisheries, including Integrated Ecosystem Assessments e.
The information that we managed to retrieve was not equally distributed among EU countries, with Spain having more detailed information at temporal and spatial scale, followed by Italy and Greece, while only few records were available for Croatia and none for other EU countries.
Most of the information derived from research and monitoring projects related to discards in the period — e. It seems that the first monitoring projects quantified discards in much detail and described an issue which had attracted little attention up to then. Undoubtedly, more historical data on discards exist in databases and technical reports that are not publicly available and were not accessible to us.
The inclusion of this information could complete the gaps in order to shed light on the evolution of trawl discards in the Mediterranean in terms of discard ratios, diversity and size structure of discards. However, given that the general patterns reported in the literature, highlighted in our analysis and discussed below i. In any case, improving access to such data would favor fisheries research needs and management in the Mediterranean. Furthermore, the use of raw data collected under the EU Data Collection Framework could help tracking the progress of the discards issue from the onset of the first monitoring programs until present, across the basin.
Our review showed that discard ratios highly fluctuated within and among species. As a result of the above, the discarded fractions of the so considered target species were usually very low or even negligible and comprised damaged or undersized specimens Carbonell et al.
This was obvious across the basin for all the main commercial species hake, red mullet, stripped red mullet, Norway lobster, deep sea pink shrimp, red shrimp of the bottom trawl fisheries considered in our analysis.
In this area discards seem to be more associated with undersized discards and local compliance with MCRS regulations. In contrast, in other areas such as GSA 9 and 11 for hake and GSA 11 for red mullets the high discard ratios reflected the concentration of the fishery on nursery areas and in the recruitment periods. Especially in GSA 11 the large discard ratios of hake, red mullet and stripped red mullet are partly attributed i to the extended presence of nurseries of these species Colloca et al.
Specifically, the majority of vessels off Sardinia exploit the deep part of the continental shelf nursery of hake as well as the slope, where the main targets are deep water pink shrimp, Norway lobster, red shrimp and giant red shrimp, Aristeomporpha foliacea Follesa et al. However, due to the inclusion in the analyses of discard ratios estimated only based on weight of catches, the ratios may not always reflect high discarding of juveniles in nursery areas e. Apart from these scarce exceptions, low ratios were the rule for target species across the basin, which additionally seemed to be sustained throughout the years.
However, most studies in the Mediterranean report relatively low proportion of key commercial i. Nevertheless, it is reported that a great amount of the bycatch is commercialized since numerous bycatch species are occasionally landed, reducing the discarded quantities to lower levels. For example, in the strait of Sicily, for 1 kg of targeted shrimps 9. Despite the commercialization of several non-target species, a large number of species that are always totally discarded are included in the catch e.
The species belonging to the commercial bycatch were usually characterized by higher discard ratios than the most valuable species and exhibited higher fluctuations geographically e. The range of the fluctuations also depended on the species, since a species which is marketed in one country or even GSA may not be marketed in the others. On the other hand, bogue, a coastal species, was more appreciated in Greece than in Spain as demonstrated by the lower discard ratio in the former; commercialization in the eastern Mediterranean is mainly oriented to human consumption while in the western part it is related with use in aquaculture.
Regional differences in the discard ratios of red pandora, which is also a coastal species, can be due to different market preferences for this species or for specific sizes. Greater forkbeard is mainly a bycatch of the deep demersal fishery, usually caught in small to intermediate biomasses and abundances and quite homogeneous discard ratios were observed in the western and eastern areas.
Regarding blue whiting, an important bycatch species without MCRS at EU level and for which discarding is due to market preferences, a quite homogeneous percentage of discards was noted, at least for the western GSAs where most of the information derived from. The three species of sharks, which are the most studied in the discards literature and the most abundant in demersal fisheries, represented a different percentage of discards, always related with small sizes.
The velvet belly lanternshark was almost always discarded across the basin but is now partially commercialized, at least in the Balearic area A. Carbonell, unpublished data. Regional and seasonal environmental differences e. The synergistic effect of such factors determines among others the size distribution e.
As a result, nursery grounds are often characterized by high discard ratios e. Further to the above, legal measures e. Nevertheless, discarding in the Mediterranean is mainly market driven and is further affected by socio-cultural traits which eventually affect market demands Tsagarakis et al. At the haul level, discards of bycatch species may be high when their catch is too low to be sold or when the catches of the target species are adequate enough to provide a high income to the fisher Tsagarakis et al.
At the end, the decision to discard or not is affected by a combination of factors which is not always easy to disentangle.
Comparing this first FAO estimate to those of Kelleher 20 for the period — and to the findings of the current study indicates a declining trend from the late s to , with the latest discard estimate being around half of the initial estimate. While this assessment of changes in estimated discard levels is compromised by different approaches being used in each of the three assessments, the temporal pattern and estimate from the current assessment are both consistent with the findings of Zeller et al.
Zeller et al. Tropical shrimp, bottom trawl and other trawl gears, long identified as having relatively high discards 7 , 20 , remain the largest opportunities for further reductions. The declines in discard levels and rates over the past few decades may have been due to: i the use of more selective fishing gear and methods that avoid the capture of unwanted species and sizes; ii increased retention of catches that had previously been discarded due to changes in market demand e.
FAO 29 hypothesized that a reduction in discards contributed to a 3. We encountered several limitations that reduced the certainty of the discard estimates, including: small sample sizes of records of observed discard rates for some gear types e. These limitations could be addressed in future studies. Furthermore, fishery-specific estimates of discard rates were not available for over two thirds of global fisheries.
Addressing the low level or lack of observer coverage in the majority of global fisheries 11 , where electronic monitoring will be especially important in fisheries where conventional human observer coverage is problematic, would improve future discard estimates as well as help achieve other critical monitoring and management objectives.
Periodic updated global discard assessments, with improvements in data quality and assessment approaches, are needed to assess the performance of discard reduction measures, where continued reductions will further reduce the wastage of natural resources and improve global food security. Discard rates and levels were estimated for global commercial marine and estuarine capture fisheries, by FAO area, gear type and target species. Discards were defined, consistent with Kelleher 20 , as the portion of the catch that is returned to the sea whole, alive or dead.
Fishery-specific discard rates were used for fisheries for which records of discard rates were available.
These global discard rates were estimated for 25 gear types with gear-specific zero-inflated Beta regression models 32 fitted within a Bayesian inferential framework using the brms interface 33 to the Stan computation engine Details on the model fitting and evaluation procedures used here can be found in Gilman et al.
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Opportunistic exploitation: an overlooked pathway to extinction.
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